1. Field of the Invention
This invention relates to a process for producing cut shapes of amorphous metal, and more particularly, to a selective etching process for producing cut laminations that are bonded together to form a generally polyhedrally shaped, low core loss, bulk amorphous metal magnetic component useful in electric motors and inductive magnetic devices.
2. Description of the Prior Art
Magnetic components made from a plurality of stacked laminations of sheet-form magnetic materials are widely used in electric motors and in inductive devices such as transformers, ballasts, inductors, saturable reactors, and the like. The magnetic material is typically selected based on both the required device properties and economic considerations.
Non-oriented electrical steel is most frequently chosen for electric motor components. In variable reluctance motors and eddy current motors, the stators are made from stacked laminations. Both the stator and the rotor are made from stacked laminations in squirrel cage motors, reluctance synchronous motors and switched reluctance motors. Each lamination is typically formed by stamping, punching or cutting the mechanically soft, non-oriented electrical steel into the desired shape. The formed laminations are then stacked and bound to form rotors or stators that have the desired geometry, along with sufficient mechanical integrity to maintain their configuration during production and operation of the motor.
The stator and the rotor in a rotating electrical machine are separated by small gaps that are either: (i) radial, i.e., generally perpendicular the axis of rotation of the rotor, or (ii) axial, i.e., generally parallel to the rotation axis and separated by some distance. In a dynamoelectric machine, lines of magnetic flux link the rotor and stator by traversing the gaps. Electromagnetic machines thus may be broadly classified as radial or axial flux designs, respectively. The corresponding terms radial and axial gap designs are also used in the motor art. Radial flux machines are by far most common. The aforesaid punching and stacking methods are widely used for constructing rotors and stators for radial flux motors.
Although amorphous metals offer superior magnetic performance when compared to non-oriented electrical steels, they have long been considered unsuitable for use in bulk magnetic components such as the rotors and stators of electric motors due to certain physical properties and the ensuing impediments to fabrication. For example, amorphous metals are thinner and harder than non-oriented steel, and consequently cause fabrication tools and dies to wear more rapidly. The resulting increase in the tooling and manufacturing costs makes fabricating bulk amorphous metal magnetic components using such conventional techniques, such as punching and stamping, commercially impractical. The thinness of amorphous metals also translates into an increased number of laminations in the assembled components, further increasing the total cost of an amorphous metal rotor or stator magnet assembly.
Amorphous metal is typically supplied in a thin continuous ribbon having a uniform ribbon width. However, amorphous metal is a very hard material, making it very difficult to cut or form easily. Once annealed to achieve peak magnetic properties, amorphous metal ribbon becomes very brittle. This makes it difficult and expensive to use conventional approaches to construct a bulk amorphous metal magnetic component. The brittleness of amorphous metal ribbon may also cause concern for the durability of the bulk magnetic component in an application such as an electric motor.
Magnetic stators are subject to extremely high magnetic forces, which vary rapidly at the frequencies needed for high rotational speed. These magnetic forces are capable of placing considerable stresses on the stator material, and may damage an amorphous metal magnetic stator. Rotors are further subjected to mechanical forces due both to normal rotation and to rotational acceleration when the machine is energized or de-energized and when the loading changes, perhaps abruptly.
A limited number of non-conventional approaches have been proposed for constructing amorphous metal components. For example, U.S. Pat. No. 4,197,146 to Frischmann discloses a stator fabricated from molded and compacted amorphous metal flake. Although this method permits formation of complex stator shapes, the structure contains numerous air gaps between the discrete flake particles of amorphous metal. Such a structure greatly increases the reluctance of the magnetic circuit and thus the electric current required to operate the motor.
The approach taught by German Patents DE 28 05 435 and DE 28 05 438 divides the stator into wound pieces and pole pieces. A non-magnetic material is inserted into the joints between the wound pieces and pole pieces, increasing the effective gap, and thus increasing the reluctance of the magnetic circuit and the electric current required to operate the motor. The layers of material that comprise the pole pieces are oriented with their planes perpendicular to the planes of the layers in the wound back iron pieces. This configuration further increases the reluctance of the stator, because contiguous layers of the wound pieces and of the pole pieces meet only at points, not along full line segments, at the joints between their respective faces. In addition, this approach teaches that the laminations in the wound pieces are attached to one another by welding. The use of heat intensive processes, such as welding, to attach amorphous metal laminations will recrystallize the amorphous metal at and around the joint. Even small sections of recrystallized amorphous metal will normally increase the magnetic losses in the stator to an unacceptable level.
Another difficulty associated with the use of ferromagnetic amorphous metals arises from the phenomenon of magnetostriction. Certain magnetic properties of any magnetostrictive material change in response to imposed mechanical stress. For example, the magnetic permeability of a component containing amorphous materials typically is reduced and the core losses increased when the component is subjected to stress. The degradation of soft magnetic properties of the amorphous metal device due to the magnetostriction phenomenon may be caused by stresses resulting from any combination of sources, including: (i) magnetic and mechanical forces during the operation of the electric motor; (ii) mechanical stresses resulting from mechanical clamping or otherwise fixing the bulk amorphous metal magnetic components in place; or (iii) internal stresses caused by the thermal expansion and/or expansion due to magnetic saturation of the amorphous metal material. As an amorphous metal magnetic stator is stressed, the efficiency at which it directs or focuses magnetic flux is reduced, resulting in higher magnetic losses, reduced efficiency, increased heat production, and reduced power. The extent of this degradation may be considerable depending upon the particular amorphous metal material and the actual intensity of the stresses, as indicated by U.S. Pat. No. 5,731,649 (“the '649 patent”). The degradation of core loss is often expressed as a destruction factor, i.e., a ratio of the core loss actually exhibited by a finished device and the inherent core loss of the constituent material tested under stress-free, laboratory conditions.
Moreover, amorphous metals have far lower anisotropy energies than other conventional soft magnetic materials, including common electrical steels. As a result, stress levels that would not have a deleterious effect on the magnetic properties of these conventional metals have a severe impact on magnetic properties important for motor components, e.g. permeability and core loss. For example, the '649 patent further discloses that forming amorphous metal cores by rolling amorphous metal into a coil, with lamination using an epoxy, detrimentally restricts the thermal and magnetic saturation expansion of the coil of material, resulting in high internal stresses and magnetostriction that reduces the efficiency of a motor or generator incorporating such a core. In order to avoid stress-induced degradation of magnetic properties, the '649 patent discloses a magnetic component comprising a plurality of stacked or coiled sections of amorphous metal carefully mounted or contained in a dielectric enclosure without the use of adhesive bonding.
A number of applications in current technology, including such widely diverse areas such as high-speed machine tools, aerospace motors and actuators, and spindle drive motors for magnetic and optical disk drives used for data storage in computers and other microelectronic devices, require electrical motors operable at high speeds, many times in excess of 15,000-20,000 rpm, and in some cases up to 100,000 rpm. The limitations of magnetic components made using existing materials entail substantial and undesirable design compromises. In many applications, the core losses of the electrical steels typically used in motor components are prohibitive. In such cases a designer may be forced to use a permalloy alloy as an alternative. However, the attendant reduction in saturation induction (e.g. 0.6-0.9 T or less for various permalloy alloys versus 1.8-2.0 T for ordinary electrical steels) necessitates an increase in the size of magnetic components comprised of permalloy or variants thereof. Furthermore, the desirable soft magnetic properties of the permalloys are adversely and irreversibly affected by plastic deformation, which can occur at relatively low stress levels. Such stresses may occur either during manufacture or operation of the permalloy component.
Inductive devices are essential components of a wide variety of modern electrical and electronic equipment, most commonly including transformers and inductors. Most of these non-rotating devices employ a core comprising a soft ferromagnetic material and one or more electrical windings that encircle the core. Inductors generally employ a single winding with two terminals, and serve as filters and energy storage devices. Transformers generally have two or more windings. They transform voltages from one level to at least one other desired level, and electrically isolate different portions of an overall electric circuit. Inductive devices are available in widely varying sizes with correspondingly varying power capacities. Different types of inductive devices are optimized for operation at frequencies over a very wide range, from DC to GHz. Virtually every known type of soft magnetic material finds application in the construction of inductive devices. Selection of a particular soft magnetic material depends on the combination of properties needed, the availability of the material in a form that lends itself to efficient manufacture, and the volume and cost required to serve a given market. In general, a desirable soft ferromagnetic core material has high saturation induction Bsat to minimize core size, along with low coercivity Hc, high magnetic permeability μ, and low core loss to maximize efficiency.
Components of small to moderate size inductors and transformers for electrical and electronic devices are also frequently constructed using laminations punched from various grades of magnetic steel supplied in sheets having thickness as low as 100 μm. The laminations are generally stacked and secured and subsequently wound with the requisite one or more electrical windings that typically comprise high conductivity copper or aluminum wire. These laminations are commonly employed in cores with a variety of known shapes.
Many of the shapes used for inductor and transformer cores are assembled from constituent components that have the general form of certain block letters, such as “C,” “U,” “E,” and “I”, by which the components are often identified. The assembled shape may further be denoted by the letters reflecting the constituent components; for example, an “E-I” shape would be made by assembling an “E” component with an “I” component. Other widely used assembled shapes include “E-E,” “C-I,” and “C-C.” Constituent components for prior art cores of these shapes have been constructed of both laminated sheets of conventional crystalline ferromagnetic metal and machined bulk soft ferrite blocks.
A significant trend in recent electronics technology has been the design of power supplies, converters, and related circuits using switch-mode circuit topologies. The increased capabilities of available power semiconductor switching devices have allowed switch-mode devices to operate at increasingly high frequencies. Many devices that formerly were designed with linear regulation and operation at line frequencies (generally 50-60 Hz on the power grid or 400 Hz in military applications) are now based on switch-mode regulation at frequencies that are often 5-200 kHz, and sometimes as much as 1 MHz. A principal driving force for the increase in frequency is the concomitant reduction in the size of the required magnetic components, such as transformers and inductors. However, the increase in frequency also markedly increases the magnetic losses of these components. Thus there exists a significant need to lower these losses.
The limitations of magnetic components made using existing materials entail substantial and undesirable design compromises. In many applications, the core losses of the common electrical steels are prohibitive. In such cases a designer may be forced to use a permalloy alloy or a ferrite as an alternative. However, the attendant reduction in saturation induction (e.g. 0.6-0.9 T or less for various permalloy alloys and 0.3-0.4 T for ferrites, versus 1.8-2.0 T for ordinary electrical steels) necessitates an increase in the size of the resulting magnetic components. Furthermore, the desirable soft magnetic properties of the permalloys are adversely and irreversibly affected by plastic deformation which can occur at relatively low stress levels. Such stresses may occur during either manufacture or operation of the permalloy component. While soft ferrites often have attractively low losses, their low induction values result in impractically large devices for many applications wherein space is an important consideration. Moreover, the increased size of the core undesirably necessitates a longer electrical winding, so ohmic losses increase.
For electronic applications such as saturable reactors and some chokes, amorphous metal has been employed in the form of spirally wound, round toroidal cores. Devices in this form are available commercially with diameters typically ranging from a few millimeters to a few centimeters and are commonly used in switch-mode power supplies providing up to several hundred volt-amperes (VA). This core configuration affords a completely closed magnetic circuit, with negligible demagnetizing factor. However, in order to achieve a desired energy storage capability, many inductors include a magnetic circuit with a discrete air gap. The presence of the gap results in a non-negligible demagnetizing factor and an associated shape anisotropy that are manifested in a sheared magnetization (B-H) loop. The shape anisotropy may be much higher than the possible induced magnetic anisotropy, increasing the energy storage capacity proportionately.
Toroidal cores made with discrete air gaps and conventional material have been proposed for such energy storage applications. However, the gapped toroidal geometry affords only minimal design flexibility. It is generally difficult or impossible for a device user to adjust the gap so as to select a desired degree of shearing and energy storage. In addition, the equipment needed to apply windings to a toroidal core is more complicated, expensive, and difficult to operate than comparable winding equipment for laminated cores. Oftentimes a core of toroidal geometry cannot be used in a high current application, because the heavy gage wire dictated by the rated current cannot be bent to the extent needed in the winding of a toroid. In addition, toroidal designs have only a single magnetic circuit. As a result, they are not well suited and are difficult to adapt for polyphase transformers and inductors, including especially common three-phase devices. Other configurations more amenable to easy manufacture and application are thus sought.
Moreover, the stresses inherent in a strip-wound toroidal core give rise to certain problems. The winding inherently places the outside surface of the strip in tension and the inside in compression. Additional stress is contributed by the linear tension needed to insure smooth winding. As a consequence of magnetostriction, a wound toroid typically exhibits magnetic properties that are inferior to those of the same strip measured in a flat strip configuration. Annealing in general is able to relieve only a portion of the stress, so only a part of the degradation is eliminated. In addition, gapping a wound toroid frequently causes additional problems. Any residual hoop stress in the wound structure is at least partially removed on gapping. In practice the net hoop stress is not predictable and may be either compressive or tensile. Therefore the actual gap tends to close or open in the respective cases by an unpredictable amount as required to establish a new stress equilibrium. Therefore, the final gap is generally different from the intended gap, absent corrective measures. Since the magnetic reluctance of the core is determined largely by the gap, the magnetic properties of finished cores are often difficult to reproduce on a consistent basis in the course of high-volume production.
Amorphous metals have also been used in transformers for much higher power devices, such as distribution transformers for the electric power grid that have nameplate ratings of 10 kVA to 1 MVA or more. The cores for these transformers are often formed in a step-lap wound, generally rectangular configuration. In one common construction method, the rectangular core is first formed and annealed. The core is then unlaced to allow pre-formed windings to be slipped over the long legs of the core. Following incorporation of the pre-formed windings, the layers are relaced and secured. A typical process for constructing a distribution transformer in this manner is set forth in U.S. Pat. No. 4,734,975 to Ballard et al. Such a process understandably entails significant manual labor and manipulation steps involving brittle annealed amorphous metal ribbons. These steps are especially tedious and difficult to accomplish with cores smaller than 10 kVA. Furthermore, in this configuration, the cores are not readily susceptible to controllable introduction of an air gap, which is needed for many inductor applications.
Notwithstanding the advances represented by the above disclosures, there remains a need in the art for improved amorphous metal magnetic components that exhibit a combination of excellent magnetic and physical properties needed for high speed, high efficiency rotating electric machines, as well as other non-rotating inductive devices. Construction methods are also sought that use amorphous metal efficiently and can be implemented for high volume production of motors of various types and magnetic components.